Thread: Communication through pipes

Originally Posted by Salem

Did you change it to

int status;

///

waitpid( -1, &status, 0 );



Because simply changing it to waitpid(-1, status, 0); would make the compiler shut up (hey, I wanted an int pointer, and this is one), but it doesn't make it right.

Each and every one of your * variables needs careful analysis to make sure it's both necessary and pointing at some valid memory.

Yes, I did change it to that and still it ain't working.
I completely have no idea why he just doesn't enter the son part instead of the dad part .. :/

From the fork(2): create child process - Linux man page docs:

Return value: On success, the PID of the child process is returned in the parent, and 0 is returned in the child. On failure, -1 is returned in the parent, no child process is created, and errno is set appropriately.

So the value of the "pid" variable is 0 in the child, and > 0 in the parent.

I think it's -1 also since this is the meaning of it :

If pid is equal to -1, the calling process waits for any child process to terminate.

Yes, that's from the waitpid(2): wait for process to change state - Linux man page page. It's just a special value meaning "any child". Your waitpid function looks ok.

I think it should be:

Code:

 pid = fork();

 if (pid == -1)
    /// error
 else if (pid == 0) 
    //execlp
  else
    // parent stuff
    waitpid()

Just so long as you move the fork() call outside of that if condition, I think it's probably fine otherwise.

What makes you think that the child is going to the parent process? Is it printing "pai" twice?
If it's just printing it once, I think it might be because of this:

Code:

if(pid==0) // filho
    {
        dup2(fd[1],1);
        printf("filho\n");

I think that dup2 has replaced the child's stdout with the pipe. You could fprintf to stderr instead to see if anything comes up.

Code:

while((d=read(1,buffer,1000))>0) {

And there you're trying to read from stdout?

Originally Posted by smokeyangel

From the fork(2): create child process - Linux man page docs:



So the value of the "pid" variable is 0 in the child, and > 0 in the parent.



Yes, that's from the waitpid(2): wait for process to change state - Linux man page page. It's just a special value meaning "any child". Your waitpid function looks ok.

I think it should be:

Code:

 pid = fork();

 if (pid == -1)
    /// error
 else if (pid == 0) 
    //execlp
  else
    // parent stuff
    waitpid()

Just so long as you move the fork() call outside of that if condition, I think it's probably fine otherwise.

What makes you think that the child is going to the parent process? Is it printing "pai" twice?
If it's just printing it once, I think it might be because of this:

Code:

if(pid==0) // filho
    {
        dup2(fd[1],1);
        printf("filho\n");

I think that dup2 has replaced the child's stdout with the pipe. You could fprintf to stderr instead to see if anything comes up.

Code:

while((d=read(1,buffer,1000))>0) {

And there you're trying to read from stdout?

I don't think the child is going to the father process.
What I think and know is my program isn't entering here :

Code:

else if (pid==0)
    {
        dup2(fd[1],1);
        printf("son\n");
        int a = execlp(cmd,cmd,arg,NULL);
        perror("demo");
    }

With :

Code:

while((d=read(1,buffer,1000))>0) {

I'm trying to read what the son has wrote using dup2(fd[1],1)

Originally Posted by DeanWinchester

What I think and know is my program isn't entering here :

Code:

else if (pid==0)
    {
        dup2(fd[1],1);
        printf("son\n");
        int a = execlp(cmd,cmd,arg,NULL);
        perror("demo");
    }

Ok. Perhaps pid is getting corrupted somehow. What does your whole function look like now?

Can you add some fprintfs to check the value of pid after the fork and before the dup2? You should see the parent and child's pids, so should be able to see if the child pid isn't 0.

Code:

fprintf(STDERR, "\npid: %d", pid);
else if (pid==0)
    {
        fprintf(STDERR, "\npid: %d", pid);
        dup2(fd[1],1);
        printf("son\n");

Code:

while((d=read(1,buffer,1000))>0) {

I'm trying to read what the son has wrote using dup2(fd[1],1)

Ah. That's not right. pipe(2): create pipe - Linux man page

Originally Posted by pipe docs

pipe() creates a pipe, a unidirectional data channel that can be used for interprocess communication. The array pipefd is used to return two file descriptors referring to the ends of the pipe. pipefd[0] refers to the read end of the pipe. pipefd[1] refers to the write end of the pipe. Data written to the write end of the pipe is buffered by the kernel until it is read from the read end of the pipe.

So the all the child's standard output will go to the pipe, To read it you need to read from fd[0].

Originally Posted by smokeyangel

Ok. Perhaps pid is getting corrupted somehow. What does your whole function look like now?

Can you add some fprintfs to check the value of pid after the fork and before the dup2? You should see the parent and child's pids, so should be able to see if the child pid isn't 0.

Code:

fprintf(STDERR, "\npid: %d", pid);
else if (pid==0)
    {
        fprintf(STDERR, "\npid: %d", pid);
        dup2(fd[1],1);
        printf("son\n");

Ah. That's not right. pipe(2): create pipe - Linux man page


So the all the child's standard output will go to the pipe, To read it you need to read from fd[0].

My whole function right now looks like this :

Code:

void map(char* cmd, char* arg){
    int i;
    int d=0;
    int pid = fork();
    FILE *stream;
    int status;
    char buffer[1000];
    stream = fopen("merdas.txt","a");
    int fd[2];
    if(pipe(fd)==-1){perror("demo");exit(1);}
    if (pid == -1)   {perror("demo"); exit(1);}
    fprintf(STDERR, "\npid: %d", pid);
    else if (pid==0)
    {
        fprintf(STDERR, "\npid: %d", pid);
        dup2(fd[1],1);
        printf("son\n");
        int a = execlp(cmd,cmd,arg,NULL);
        perror("demo");
    }
    else
    {
        printf("pai\n");
        while((d=read(fd[0],buffer,1000))>0) {
        buffer[d] = '\0';
        printf("<<<<%s\n",buffer);
    }
    waitpid(pid, &status, 0);


    }
}

I've changed the value '1' to fd[0] on read, thanks for pointing that out.

About the fprintf's, I won't be able to get the value from the second fprintf since when I run my program it never enters the (pid==0) section.

I'm not being able to use that first printf aswell because I get this error :

soluis2.c:142: error: expected expression before ‘else’

Thanks!

Thanks.
It's printing the pids as following :

pid: 1355
pid: 0
pid: 0

On the father, it's already reading two lines from the the exec output :

Code:

printf("<<<<%s\n",buffer);

With this printf I'm already obtaining two lines.

The exec output has around 15 lines though, any idea why it's not printing the rest ?

Alright, my mate managed to get it working!

Thanks so much!